Role of interlayer coupling in second harmonic generation in bilayer transition metal dichalcogenides

Little is known about the role of weak interlayer coupling in the second harmonic generation (SHG) effects of two-dimensional van derWaals (vdW) systems. In this paper, taking homobilayerMoS(2)/MoS2 and heterobilayer MoS2/MoSe2 as typical examples, we have systemically investigated their SHG susceptibilities chi((2)) as a function of interlayer hopping strength (t(int)) using first-principles calculations. For the chi((2))(yyy) (0; 0, 0) of both MoS2/MoS2 and MoS2/MoSe2, although the increase of tint can increase the intensities of interlayer optical transitions (IOT), the increased band repulsion around the Gamma point can eventually decrease their chi((2))(yyy) (0; 0, 0) values; the larger the t(int), the smaller the chi((2))(yyy) (0; 0, 0). For the vertical bar chi((2))(yyy) (-2 omega; omega, omega)vertical bar spectra of MoS2/MoSe2 in the low photon-energy region, opposite to the MoS2/MoS2, their peak values are very sensitive to the variable tint, due to the strong t(int)-dependent IOT dominating in the band edge; the larger the t(int), the larger the vertical bar chi((2))(yyy) (-2 omega; omega, omega)vertical bar. For the vertical bar chi((2))(yyy) (-2 omega; omega, omega)vertical bar of MoS2/MoS2 in the high photon-energy region, comparing to the MoS2/MoSe2, their peak values will decrease in a much more noticeable way as the t(int) increases, due to the larger reduction of band-nesting effect. Our study not only can successfully explain the puzzling experimental observations for the different SHG responses in different bilayer transition metal dichalcogenides under variable t(int), but also may provide a general understanding for designing controllable the SHG effects in the vdW systems.

Automated summary

This study investigates the role of weak interlayer coupling in the second harmonic generation (SHG) effects of two-dimensional van derWaals systems using first-principles calculations. The results show that the interlayer hopping strength has a significant impact on the SHG susceptibility, indicating a potential controllability for SHG effects.